1. Field of the Invention
This invention relates to an artificial lift system for removing fluid from an underground formation, and more specifically to an augmented artificial lift system utilizing pressure reduction to increase the efficiency of the artificial lift system.
2. Description of Related Art
Artificial lift systems are commonly used to extract fluids, such as oil, water and natural gas, from underground geological formations. Often times, the formations are more than 1,000 feet below the surface of the earth. The internal pressure of the geological formation is often insufficient to naturally raise commercial quantities of the fluid or gas from the formation through a bore hole. When the formation has a sufficient internal pressure to naturally lift the fluid from the formation, the natural pressure is often inadequate to produce the desired flow rate. Therefore, it is desirable to artificially lift the fluid from the formation by means of an artificial lift system.
Typically, the formation can comprise several separate layers or strata containing the fluid or can comprise a single large reservoir. A bore hole is drilled into the earth and passes through the different layers of the formation until the deepest layer is reached. Due to economic considerations many bore holes extend only to the deepest part of the formation. In certain applications it is desired to extend the bore hole beyond the bottom of the formation. The portion of the bore hole that extends beyond the bottom of the formation is known as a "rat hole." The location and depth of the bore hole is carefully controlled because of the great expense in drilling the bore hole.
After the bore hole is drilled, the bore hole is lined with a casing substantially along its entire length to prevent collapse of the bore hole and to protect surface water from contamination. However, the bore hole is often only lined with the casing to the top of the gas and fluid containing formation leaving the lower section of the bore hole uncased. The uncased section is referred to as an open hole. The casing is cemented in place and sealed at surface by a wellhead and can have one or more pipes, tubes or strings (metal rods) disposed therein and extending into the bore hole from the wellhead. One of the tubes is typically a production tube, which is used to carry fluid to the surface.
Currently, many different types of artificial lift systems are used to lift the fluid from the formation. The most common artificial lift systems are: progressive cavity pumps, beam pumps and subsurface gas lift (SSGL). A progressive cavity pump is relatively expensive, approximately $25,000, to install but can deliver relatively large volumes of fluid and remove all the fluid from the formation. A progressive cavity pump comprises an engine or electric motor driven hydraulic pump connected to a hydraulic motor mounted on the top of the wellhead and connected to a hydraulic pump at the bottom of a production tubing. The hydraulic motor turns a rod string that is connected to a pump rotor, which turns with respect to a pump stator. The pump rotor is helical in shape and forms a series of progressive cavities as it turns to lift or pump the fluid from the bottom of the casing into the production tube and to the surface. Although the progressive cavity pump is satisfactory in raising fluid from the formation, the hydraulic pump system requires a containment building and liner in the event of an oil leak. The possibility of an oil leak in the progressive cavity pump system also raises environmental concerns because many of the bore holes are drilled in environmentally sensitive or wilderness areas. The progressive cavity pump also requires, in certain applications, at least 100 feet of a rat hole, which adds extra cost. Of the previously mentioned artificial lift systems, the progressive cavity pump has the highest maintenance costs and greatest amount of down time requiring rig service. A soft seal stuffing box seals around the rotating rod string and must be lubricated daily and acoustic annular fluid levels must be obtained at regular intervals to ensure that the fluid is adequately high above the pump and that it does not run dry and destroy itself.
A beam pump is also relatively expensive, approximately $15,000, to install but can also remove all the fluid from the formation. The beam pump comprises a pivotally mounted beam that is positioned over the wellhead and connected to a rod string extending into the production tube in the bore hole. The lower end of the rod string is connected to a pump disposed near the bottom of the bore hole. The beam pump is operated by a gas engine or an electric motor. If an electric motor is used, it is necessary to run power lines to the beam pump because many of the beam pumps are placed in remote wilderness areas. The beam pump has several disadvantages. First, there are many environmental concerns. There may be leakage in the engine or gear box of the power source, requiring construction of a containment area. Further, if an electric motor is used in place of the gas engine, it is necessary to run a power line to the electric motor, which often destroys or degrades the surrounding environment. The beam pump, like the progressive cavity pump, has several components that require regular lubrication. The beam pump also uses a soft seal stuffing box to seal around the reciprocating rod string.
The subsurface gas lift (SSGL) is the least expensive artificial lift system to install, approximately $7,500. The SSGL uses pressurized gas carried by a separate tube from the surface to the lower end of a production tube to raise fluid in the production tube upon injection of the pressurized gas. The production tube usually has a one-way valve at its lower end so that fluid standing in the formation can enter the production tube and rise in the production tube to the level of fluid in the formation. The SSGL can be used with or without a plunger disposed within the production tubing. The SSGL is the most environmentally friendly and maintenance free of the three commonly used artificial lift systems. Unlike the other artificial lift systems, the subsurface gas lift system requires no systematic lubrication of the gas regulator and the motor valve. The SSGL maintains greater integrity of the well head in controlling the possibility of fluid leaks because the well head components are hard piped with no friction oriented soft seal such as is found in the stuffing boxes of the progressive cavity and beam pumps. The SSGL is virtually silent during operation and has relatively little surface equipment compared to a beam pump or progressive cavity pump. Therefore, it has less audible and visual impact on the surrounding environment. The greatest disadvantage of the SSGL is that it becomes less efficient as more and more fluid is drawn from the formation. The SSGL can only raise the column of fluid in the production tubing. The column of fluid in the tubing is equal to the level of fluid in the formation. As more and more fluid is removed from the formation, the level of fluid in the production tubing decreases and a continuously smaller and smaller amount of fluid is raised for substantially the same amount of energy.
As the fluid level in the subsurface gas lift system decreases, there becomes a point where it is no longer cost effective, operationally safe or productive to use the subsurface gas lift system. Often times, the subsurface gas lift system is replaced with a beam pump, and its accompanying undesirable attributes. Optionally, a "rat hole" can be bored with the bore hole in a subsurface gas lift system so that most of the fluid can be raised from the formation by placing the gas injection below the level of the formation and in the rat hole. However, hundreds of bore holes were drilled without rat holes before artificial lift became a generally accepted method of production and the cost associated with boring a rat hole is such that most companies still prefer to drill little, if any, rat holes.
Another disadvantage that is common to all artificial lift systems in that as the fluid level decreases the system becomes operationally more difficult to efficiently control without damaging itself. In the event of no fluid level, the progressive cavity will quickly torque up and seize the down hole pump or twist off the rod string. The beam pump will begin to pound as gas is drawn into the pump. The end result of which will be a scored pump barrel and eventually a parted rod string. The SSGL may "dry cycle". A condition where the plunger arrives at the surface and bottom of the well with possible damaging velocity. The damage to the progressive cavity and the beam pumps will require a work over rig for repairs. The damage to the SSGL seldom requires more than a small wire line truck for a few hours to retrieve and repair the damaged components. Each of these systems, if controlled improperly, can have catastrophic failures that can be physically dangerous to the operator and can inflict environmental damage.
Therefore, it is desirable to have a cost effective artificial lift system and process for a well that are relatively environmentally safe, low maintenance, operationally predictable, easy to control and which has an acceptable level of efficiency.